Integrated circuits (IC) are typically housed within a package that is mounted to a printed circuit board (PCB). The package has conductive leads or pins that are soldered to the PCB and coupled to the IC by a lead frame. One kind of package commonly referred to as a ball grid array (BGA) is an integrated circuit package which has a plurality of solder balls that interconnect the package to a PCB. The solder balls are attached to a polyimide based flexible circuit board which has a number of conductive traces and accompanying solder pads. The integrated circuit die is connected to the solder pads of the flexible circuit by wire bonds and electrically coupled to the solder balls through conductive traces routed across the flexible circuit.
FIG. 1A illustrates a conventional BGA package 100 that includes a substrate 105 with a die 115 (i.e., an IC chip) attached to the substrate 105 using a die attach adhesive 110. The substrate 105 contains embedded multiple horizontal layers of metal conductors (not shown) connected by vertical conductors (not shown). The die 115 is positioned over substrate 105. Inputs/outputs (I/Os) of the die 115 are connected to these embedded conductors by bonding wires 120, 122. Solder balls (e.g., ball 130) are attached to a bottom surface of the substrate 105 and function as the I/O pins of the BGA package 100. A top surface of the BGA package 100 is encapsulated or overmolded by an epoxy molding (e.g., resin-based) compound 140, for example, by a transfer molding process. In its final form, the package 100 is soldered to a PCB (not shown).
One problem with conventional plastic BGA packages is that they are prone to warping, as illustrated in FIG. 1B. In conventional plastic BGA packages, warpage of substrate 105 occurs after the overmolding process. This warpage poses a problem in placement and attachment of the solder ball joints to a PCB. Overmolded structures, such as the conventional BGA package 100 shown in FIG. 1B, have a mismatch in their CTE (Coefficients of Thermal Expansion) between the epoxy molding compound 140 and the substrate 105. This CTE or thermal mismatch creates interfacial stress as the overmolded structure is cooled to room temperature from elevated processing temperatures, such as during the die attach, wire bonding and overmolding processes. The presence of the mold compound encapsulant 140 on only one side of the substrate 105 creates an unbalanced situation and leads to stress on the substrate 105, resulting in warpage. Warpage creates reliability concerns because it interferes with solder ball attachments and also impacts the integrity of other interfaces present in the package, such as the interface between the mold compound 140 and substrate 105.